A typical cooling system expansion tank is a closed vessel which, when the engine is at rest, is only partially filled with liquid coolant, the remainder of the space above the liquid being available for the volumetric expansion of the coolant due to heat. Coolant discharged from the engine flows into the tank above the level of liquid coolant via an inlet port and returns from the bottom of the tank via an outlet port to join the flow of coolant returned to the engine. Such an expansion tank also serves as a means of enabling gases dissolved or trapped in the coolant to rise to the liquid surface and escape hence they are often referred to as a degas tank. Furthermore, air above the liquid surface becomes heated by the incoming coolant, thereby further helping to pressurise the cooling system and prevent cavitation in the pump used to circulate the coolant.
It is a problem with known expansion tanks that under certain conditions coolant can flow back via the outlet port into the tank thereby filling the tank with coolant thereby eliminating or significantly reducing the expansion volume into which gas liberated from the coolant can expand. It is a further problem with such a prior art expansion tank that if the pressure cap is mistakenly removed from a filler neck of the expansion tank while the engine is hot a considerable amount of coolant will be lost as the sudden reduction in system pressure will cause extensive coolant boiling within the system which forces coolant out of the filler neck.